1. Field of the Invention
This application relates to automated lensmeters and particularly to a lensmeter which has been adapted for detecting dispersion of high index of refraction lens and the measurement of lens power of contact lens at varying radii for determining spherical aberration inherent in contact lens.
2. Statement of the Problem
Automated lensmeters have an obvious advantage over their human operated counterparts. When these instrument operate properly, they are not subject to the human factors of error. In short, the machines do not become tired and are never bored. They complete their automated tasks giving constantly consistent measurement.
Two areas of measurement have caused problems for such automated lensmeters. First, there is the problem caused by the increasing use of high index glasses with their accompanying chromatic optical dispersion (hereinafter dispersion). Secondly, the measurement of contact lenses is accompanied by spherical aberration.
Regarding the use of high index glass, cataract operations and certain malformations (and diseases) of the eye require the use of relatively high optical prescriptions in the range of 10 diopters. Where such high optical prescriptions are utilized, it is common to use high index glass. Such high index glass has the beneficial result of enabling relatively thin lenses to generate the relatively high power optical corrections required. For example, over ordinary Crown glass, a high index glass can have a thickness reduced by as much as 60%.
Unfortunately, such high index glasses have the undesirable effect of having relatively high levels of dispersion. Dispersion is the chromatic dependent property of the glass to deflect to varying degree optical wavelengths of differing colors. Unfortunately, in the case of automated lensmeters, this property of dispersion can lead to optical error in the obtained measurement for high-power optical lenses.
The sources of these errors can be easily understood. Typically, automated lensmeters utilize essentially monochromatic light sources to effect measurement. For example, measurements can be taken in the red (660 nanometer wavelength) or the near infrared (880 nanometer wavelength). Utilizing these particular wavelengths, deflections in sphere, cylinder, prism and axis are determined.
It goes without saying that the human eye does not observe in the same essentially monochromatic wavelengths utilized by such optical instruments. Specifically, observation of the human eye occurs in so-called "white" light. International standards therefore reference the measurement of the deflection of eyeglass lenses to performance at wavelengths differing from the sampling wavelengths of the lensmeters. These wavelengths are either in the yellow (587 nanometers) or green (546 nanometers).
When such automated lensmeters are initially calibrated with "standard" lenses, adjustment for the difference between the sampling wavelength and the so-called "white" light standard applicable to ordinary human observation occurs naturally in the calibration procedure. By the expedient of having the automated lensmeter "remember" in its measurement protocol how relatively regular optical prescriptions deflect light when sampled at the discrete sampling frequencies, accurate and fast determination of the prescriptions of normal range suspect optical elements can be made.
Unfortunately, when so-called high-power optical corrections are measured--especially when high index glasses are utilized--the ordinary offsets between the measuring color of the instrument light source and the actual color dependent deflection of the lens increases. Further, since the high index optical glasses change the dispersion relationship between the sampling color and the monochromatic sampling light, the ultimately determined optical prescription can contain error. Such error can be in the range of 0.1 diopter--an unacceptable error for accurate vision. Accordingly, there is a need to measure and determine dispersion and to use the measured dispersion to correct the final prescription generated by the instrument.
Automated lensmeters are also finding use in measuring the optical powers of contact lenses. Contact lenses--as worn on the eye--typically have relatively high optical curvatures which enable the lens to be captured to and essentially rest on the moist surface of the eye. These high optical curvatures change as the size of the human eye changes. As is well known to all who wear contact lenses, different size eyes require different curvatures for satisfactory lens fitting.
When contact lenses are properly fitted and properly positioned on the eye of a wearer, the relatively high curvatures utilized in fitting the lenses to the eye do not appreciably effect the ultimate optical prescription of the central portion of the lens. Specifically, and because of the lack of an air to lens interface on the negative surface of the eye where the eye and lens meet, spherical aberration is essentially absent in so far as the prescribed patient is concerned.
Unfortunately, this is not the case when the power of such lenses are determined by a lensmeter. The eye-to-lens interface at the negative surface of the contact lens is missing. In the place of the eye-to-lens interface there is substituted an air-to-lens interface. And with the introduction of this air-to-lens interface, spherical aberration is introduced into the automated lensmeter reading.
Spherical aberration with respect to such contact lenses can be easily understood. Assume that an accurate optical measurement could be made along the central axis of a contact lens. This central axis measurement can be referred to as a paraxial measurement. When, however, a highly curved lens is measured at points other than its optical center, the power of the lens changes as a function of the of the point of actual lens measurement from the lens center. This change in lens power is a function of the curvature of the lens. This change is referred to as the spherical aberration of the lens. Since this aberration is present only when the lens is measured in air, and absent when the lens is worn on the eye, determination and elimination of this effect from the measured power of a contact lens is highly desireable.
Unfortunately, and in order to determine the full power of a lens, measurement of the deflection of the lens must occur at more than one point on the lens. Indeed, for an accurate measurement to be made of a lens in sphere, cylinder and axis, measurements must be made at least three points through the lens. Further, it is common to make such measurements in as many as four points on the lens. Naturally, only one of these points can be at the paraxial or central portion of the lens. The remaining measurement points simply have to be removed from the center of the lens. And when such removal from the center of the lens occurs, the effects of spherical aberration are inevitably introduced into the measurement.
It will be understood that the understanding of the problem to be solved can constitute at least a portion of the following invention. Accordingly, in so far as understanding of the problem to be solved can constitute invention, invention is claimed.